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Optical "Edge of field" Performance

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#1 Alnitak2009

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Posted 10 November 2020 - 08:45 PM

Hi all,

 

I own a pair of 7x50 Celestron Ultimas and while they show fairly nice pinpoint stars in the center, at about a 1/3 of the field edge is distorted. I recently looked through a pair of Oberwerks 9x60's and they had about the same distortion as the C-Ultimas. While looking at a star field through these binoculars in the center sweet spot, the edge of field distortion is hard to discern because that area is using the eye's per peripheral vision. But if I center my eye towards the edge I then see the distortion. Do higher quality binoculars have better correction (flat field) or is this just the nature of the optics? Zeiss, APM, etc?

 

Anyone know of any discussions/articles on edge of field comparisons between brands.

 

Thanks for any info

Don

 

 



#2 KBHornblower

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Posted 10 November 2020 - 09:23 PM

That looks like trying to get a wide apparent field of view with simple eyepieces that cannot deliver the goods at the edge.  I saw a discussion a few weeks ago in which someone pointed out that the necessary eyepieces would be awkwardly large for binoculars.


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#3 harbinjer

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Posted 11 November 2020 - 12:48 AM

Well the Fuji Polaris series and Nikon Prostars use a field flattener lens to fix some of the issues, and a more complicated eyepiece.  I wouldn't count on the Zeiss(or Leica or Swaro) being perfect either because it's not as important to birders.  I do think one of the Swarovski models(maybe EL?) does have really good edges though. Also some binoculars will sacrifice by having a narrower field, but having it all be fairly sharp(Takahashi Astronomers come to mind).


Edited by harbinjer, 11 November 2020 - 12:48 AM.

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#4 PEterW

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Posted 11 November 2020 - 07:11 AM

Expensive models can deliver sharp to the edge, though you pay for the capability, which gets harder the wider the field of view.

Peter

#5 Jon Isaacs

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Posted 11 November 2020 - 08:51 AM

That looks like trying to get a wide apparent field of view with simple eyepieces that cannot deliver the goods at the edge.  I saw a discussion a few weeks ago in which someone pointed out that the necessary eyepieces would be awkwardly large for binoculars.

 

:waytogo:

 

Binoculars are fast, usually under F/4.  Truly sharp to the edge at F/4, that's TeleVue territory.  Decent eye relief, that's a big eye lens.  Think Delos, maybe Morpheus.

 

Binoculars are also short focal length, that means field curvature and a field flattener, more optics and it all has to be compact, it's going to be be expensive and still somewhat compromised unless you go the Nikon WX route.

 

Then you and up with 10x50s that weigh 5.5 lbs.

 

Jon


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#6 Alan French

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Posted 11 November 2020 - 09:32 AM

The further complications are that our eyes do not work well with a large pupil opening, and, even with a small pupil opening, visual acuity drops rapidly away from the center of our visual field. (Try reading some large print without looking directly at it). 

 

Clear skies, Alan


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#7 Jon Isaacs

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Posted 11 November 2020 - 10:38 AM

The further complications are that our eyes do not work well with a large pupil opening, and, even with a small pupil opening, visual acuity drops rapidly away from the center of our visual field. (Try reading some large print without looking directly at it). 

 

Clear skies, Alan

 

waytogo.gif

 

I think that actually is a benefit for binoculars, at least hand held binoculars. If I'm looking straight ahead, the aberrations are less apparent because they are in my peripheral vision.

 

With hand held binoculars, rather than scanning and roaming the field with my eye, I just keep looking straight ahead and scan with the binos. The only time I concentrate my gaze on the edge is when I'm wondering "how sharp off-axis are these binos?"

 

With a telescope, I do scan the field with my non-peripheral (central averted) vision so the aberrations are much more apparent and more distracting. So what I find acceptable/not distracting in a binocular wouldn't pass muster in a telescope.

 

I think that's why my 10x50 Ultraviews can provide pleasing views and still be compact and of a manageable weight as well as being affordable.

 

Jon


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#8 Rich V.

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Posted 11 November 2020 - 12:28 PM

Don, I had a 10x50 Ultima many years ago and while I enjoyed its very good central sharpness, I noticed the sharpness dropped off quickly away from the center of the field just like your 7x50s.  That might have been excusable if it were a wide field design but a 50° AFOV bino is hardly that.

 

It wasn't until I got my Fujinon 16x70 FMTs some years later that I came to appreciate a WF bino (64°AFOV) that was essentially sharp to nearly the edge of field.  Since then, I've found the KUO MS and MS ED binos sold by APM and several other vendors also have a flat field design that works very well for me, but with greatly improved eye relief over the Fujis.  

 

Since I enjoy mounted, larger, higher mag binos over hand held scanning, a flat field is a very useful design feature that allows the whole FOV to be examined as I would with a telescope.  Your personal viewing style dictates how important the extra field sharpness is for you.

 

Rich



#9 Tamiji Homma

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Posted 11 November 2020 - 02:05 PM

Here is side-by-side photo of Nikon Prostar 7x50 (7.3 degrees TFOV) and Nikon WX 7x50 (10.7 degrees TFOV).

 

Both give flat and near-perfect star image at near edge.  Visual impact is *VERY* different.  

They spent years of research and development to satisfy their engineers' dream to have the view...

 

Yes, it comes with large size and high cost.  I wish they could make 7x35 version so that I could hand-hold it :)

 

large.jpg

 

Eyepiece side:

large.jpg

 

Tammy


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#10 Steeveaux

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Posted 14 November 2020 - 11:04 AM

waytogo.gif

 The only time I concentrate my gaze on the edge is when I'm wondering "how sharp off-axis are these binos?"

 

Bingo!

 

I think it is more important to worry about an overall flat field that doesn't "roll" as you pan.

 

Steve O.


Edited by Steeveaux, 14 November 2020 - 11:08 AM.


#11 KennyJ

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Posted 14 November 2020 - 02:53 PM

In Bill Cook's excellent, unique 2017 book, Binoculars: Fallacy & Fact, the title of chapter 19 on page 50 is:

 

"IT'S TACK SHARP FROM THE CENTRE TO THE EDGE."

 

One valid point which Bill explains may well be relevant to this discussion:

 

quote :<< The observer may see a "crystal clear" image in the centre of field, then, on seeing an equally crisp image at the edge,  supposes all points in between must provide images of equal sharpness, all the time.

But to the brain, it just isn't so. >>

 

As I suspect has already been perhaps more subtly insuated even earlier in this thread by one or two previous contributors, to refocus the eyes so as to concentrate on the edge of a binocular's field of view as opposed to the centre requires millisecond, miniscule eye movements which the observer naturally tends not to pay any attention to.

 

Kenny



#12 gwlee

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Posted 14 November 2020 - 10:04 PM

Well the Fuji Polaris series and Nikon Prostars use a field flattener lens to fix some of the issues, and a more complicated eyepiece.  I wouldn't count on the Zeiss(or Leica or Swaro) being perfect either because it's not as important to birders.  I do think one of the Swarovski models(maybe EL?) does have really good edges though. Also some binoculars will sacrifice by having a narrower field, but having it all be fairly sharp(Takahashi Astronomers come to mind).

 

Hi all,

 

I own a pair of 7x50 Celestron Ultimas and while they show fairly nice pinpoint stars in the center, at about a 1/3 of the field edge is distorted. I recently looked through a pair of Oberwerks 9x60's and they had about the same distortion as the C-Ultimas. While looking at a star field through these binoculars in the center sweet spot, the edge of field distortion is hard to discern because that area is using the eye's per peripheral vision. But if I center my eye towards the edge I then see the distortion. Do higher quality binoculars have better correction (flat field) or is this just the nature of the optics? Zeiss, APM, etc?

 

Anyone know of any discussions/articles on edge of field comparisons between brands.

 

Thanks for any info

Don

For that reason, I use binoculars that incorporate a “field flattener” in the eyepieces to correct for field curvature.  Fujinon FMT-SX, Nikon Premier, and Swarovski EL are examples. 


Edited by gwlee, 14 November 2020 - 10:05 PM.


#13 Erik Bakker

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Posted 16 November 2020 - 11:29 AM

Tammy,

 

Beautiful pictures of the Nikon Prostar and WX 7x50's side by side.

 

Do you find your Prostars still worth to have and enjoyable to observe with, given you now also have the WX 7x50? I can imagine the Prostars are still the best 7x50 of the classic porro design with different qualities that even the current state of the art WX's do not possess. What is your take on that?  



#14 Tamiji Homma

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Posted 16 November 2020 - 04:48 PM

Tammy,

 

Beautiful pictures of the Nikon Prostar and WX 7x50's side by side.

 

Do you find your Prostars still worth to have and enjoyable to observe with, given you now also have the WX 7x50? I can imagine the Prostars are still the best 7x50 of the classic porro design with different qualities that even the current state of the art WX's do not possess. What is your take on that?  

 

Hi Erik,

 

Yes, absolutely worth owning and enjoying.  I use Prostar 7x50 much more than WX 7x50.

 

I think Prostar was released in early 80's.  It has been one of the best flat field 7.3 degrees binoculars.

Yes, it is narrow AFOV but I am used to it and I am happy with anything that gives different view from naked eyes :)

 

Stay safe.

 

Tammy


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#15 Erik Bakker

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Posted 16 November 2020 - 05:33 PM

Good to hear Tammy and wishing you safety too!

 

What stands out in the views and use of the Prostar that makes you take them out and enjoy often?



#16 Tamiji Homma

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Posted 17 November 2020 - 11:12 AM

Good to hear Tammy and wishing you safety too!

 

What stands out in the views and use of the Prostar that makes you take them out and enjoy often?

 

Hi Erik,

 

There are a few reasons that I like about it.  I like the classic Porro style binoculars in my hands.  It feels good :)

 

The view that I like is consistent view across field.  It is the first binoculars to provide such consistent (flat and sharp across field) that I had.  So I take the binoculars every time I go out with telescope while I am waiting for scope to cool, sitting on star chair.  It calms me down after busy work :)

 

Tammy


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#17 Erik Bakker

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Posted 17 November 2020 - 01:29 PM

Great views of the heavens are balm for the soul indeed Tammy!


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